The present invention relates to optical projection using diffraction for uses such as three dimensional (3D) surface measurements for facial recognition or other purposes.
Optical projection of a pattern is used in applications such as 3D surface measurements. The positions of a pattern of dots caused by beams projected onto a flat surface can be determined. When the same pattern of dots is projected on a 3D surface to be measured, the positions of the dots will deviate from their designed positions as a result of the different intersection height on the 3D surface. These deviations can be measured and correlated to the different distances, or depth, of the 3D surface, and a 3D image can be generated.
Miniature optical projectors are used to do such 3D mapping (also known as depth mapping). U.S. Patent Application Publication 2008/0240502 uses a light source (e.g., laser diode or LED) to illuminate a pattern on a transparency and project the pattern onto the object. An image detector then captures an image of the pattern that is projected onto the object. In one version, as described in International Publication WO 2008/120217, an array of micro-lenses is placed behind the transparency pattern to improve the signal contrast of the projected pattern.
In order to put an optical projector into a smartphone, it needs to be very miniaturized. One approach investigated was to use a single laser beam and a diffractive optical element (DOE) to produce a dot pattern. But in order to generate a pattern with enough dots, the ratio of the 0th order beam to the diffracted beams was too large. For measurements of a user's face, a large 0 order beam is not acceptable because of the potential damage to the user's eyes.
Prime Sense realized that if they divide the laser beam into M beams and use M DOEs the ratio between the 0th order beam and the diffracted beams can be minimized by factor of M. This approach is described in U.S. Patent Application Publication 2009/0185274, and uses two diffractive optical elements (DOEs). In one embodiment, the first DOE acts as a beam-splitter which splits the emitted beam into a multiple beams, and the second DOE serves as a pattern generator to form a diffraction pattern on each of the beams. A design issue is that it take space to separate the M beams so that they can illuminate M DOEs. This limits how much the optical projector can be miniaturized.
To provide further miniaturization, Prime Sense U.S. Patent Application Publication 20140376092 uses another approach, with a VCSEL array of laser emitters to produce a pattern, instead of using a single laser. A lens projects the non-collimated pattern to a single DOE which is used to produce multiple replicas of the pattern.
FIG. 1 shows an example application of a miniaturized optical projector for depth measurements. A smartphone 10 includes a display 12, a camera 14 and an internal processor and other electronics. Display 12 can be used for presenting information to a user, and also functions as a touch screen for inputting information. An optical projector/detector module 16 is provided. Projector/detector module 16 projects an IR image which diverges as shown by arrows 18. The IR image is projected onto a user's face 20 as a series of dots 22. A detector in optical projector/detector module 16 then detects the dots 22, and from their relative positions, can determine the depth of the various parts of the user's face 20. By combining this with traditional two dimensional facial recognition, a user's face can be detected with great accuracy.
FIG. 2 illustrates how a diffraction grating, known in the prior art, provides multiple beams that can form a pattern, such as a series of dots. A laser beam 101 is incident on the diffraction grating 102. Beam 101 is diffracted into a series of beams, such as beams 103, 104, 105, 106 and 107 as shown emerging from grating 102. The diffracted beams are described by diffraction order, with the 0 order (beam 105) being straight on the path of the original beam, then 1st order beams on each side of a one dimensional grating, then the 2nd order beams, etc. There would typically be additional diffraction orders beyond what is shown in FIG. 2. However, the intensities of beams beyond the first few orders are generally relatively weak. Most of the laser energy is distributed among the 5 major beams. Since the purpose of this particular diffraction grating is to divide a single laser beam into multiple beams, it is sometimes called a multiple beam grating (MBG).
FIG. 3 illustrates one example of how a prior art multiple beam grating (MBG) is used. An object 201 is placed in front of lens 202. Lens 202 projects an image of the object 201 to a plane 204. A multiple beam grating 203 is placed after lens 202. Grating 203 creates multiple images of the input object 201 at plane 204. If the object is the form of a “K” (205), grating 203 will duplicate this to provide a projection of 9 K images 206 as shown. This is the principle used in U.S. Patent Application Publication 2014/0376092, referenced above, to project dot patterns for 3D pattern recognition.
FIG. 4 is from U.S. Patent Application Publication 2014/0376092. An array of vertical cavity surface emitting lasers (VCSEL) 20 is placed in front of a lens 46. On top of the lens 46 is a multiple beam grating (MBG) 44. This integrated module is able to project multiple images of the VCSEL array into space to illuminate the object for 3D recognition. An integrated optical projection module 30 contains the VCSEL array 20. VCSEL die 22 is mounted on a sub-mount 32, with appropriate electrical connections 34, 36, 38. Optics 40, including projection lens 46, are mounted over the die on suitable spacers 42. Lens 46 collects and projects an output beam 50 of the VCSEL emitters. A multiple beam grating (MBG) 44, supported by thin spacers 48, creates multiple replicas 52, 54, 56 of the pattern of the lines of the VCSEL array, fanning out over a predefined angular range.
It is desirable to have an improved optical projection module which is both miniaturized and more economical to manufacture.